This invention relates to a discharge lamp lighting device and, in particular, to a device for lighting a discharge lamp and including an electronic ballast having a function of improving input current distortion.
In an aspect of the discharge lamp lighting device of the kind referred to, the device is provided with a DC power source of which a voltage fluctuates, such as a pulsating flow power source of a commercial AC source power rectified by a full-wave rectifier, and includes a boost converter as a first power converting circuit, a buck converter as a second power converting circuit, and a discharge lamp as a load. In the first power converting circuit, more specifically, an inductor is connected at one end to a positive polarity terminal of the DC power source, and at the other end through a switching element to a negative polarity terminal of the DC power source, and a diode is connected at its anode to a node of the inductor and the switching element, while the cathode of this diode is connected to a positive polarity terminal of a bulk capacitor which is connected at a negative polarity terminal to a positive polarity terminal of the DC power source. The switching element can be ON/OFF controlled by a control circuit. When this switching element is ON, a current flows from the DC power source through the inductor and switching element, and an energy is accumulated in the inductor. When the switching element is OFF, on the other hand, an electromotive force due to the energy accumulated in the inductor is superposed on a voltage of the DC power source and is charged through the diode in the bulk capacitor. On this account, the bulk capacitor is charged up to above a peak voltage of the DC power source. A boost converter as a first power converting circuit is constituted with this arrangement.
The DC voltage of the bulk capacitor is dropped by the buck converter as the second power converting circuit, and is applied to the load discharge lamp. The buck converter represented by an equivalent circuit is connected at one end of the switching element to the ground level to be able to easily drive the switching element. The switching element is connected at one end to the negative polarity terminal of the bulk capacitor and at the other end through the anode and cathode of the diode to the positive polarity terminal of the bulk capacitor An end of the inductor is connected to the anode of the diode. The other end of the inductor is connected through a parallel circuit of a capacitor and the load discharge lamp to the positive polarity terminal of the bulk capacitor.
The switching element of the buck converter is ON/OFF controlled by the control circuit, and this control circuit includes means for detecting a voltage across the switching element. The switching element will be turned ON at a timing at the minimality voltage across the switching element in OFF period of the switching element. One of the discharge lamp lighting devices of this kind has been disclosed in Japanese Patent Application No. 11-117066.
The inductor current is positive direction at the term in which a current acting similar to an ordinary buck converter. The current is negative direction at the other term in which a resonance current for performing a zero volt switching upon turning ON of a main switching element. It is a feature of this device. A charge of parasitic capacitance of the switching element and the capacitor connected in parallel are discharged with this negative directional resonance current. Whereby any short-circuit loss upon turning ON of the switching element can be restrained. The circuit efficiency can be improved in contrast to any known converter of critical continue current mode control (a measure for erasing any quiescent period in the inductor current and restraining any peak current, by turning ON the switching element simultaneously with returning to zero of the inductor current in OFF period of the switching element).
With the foregoing known arrangement, the circuit efficiency can be improved without increasing the number of the switching element more than that in the ordinary buck converter. It is necessary to satisfy predetermined conditions in the relationship between the source voltage and the load voltage. It is impertinence, apply to this arrangement to the discharge lamp lighting device in which the load voltage fluctuates. When in particular this arrangement is employed in current limiting means of a discharge lamp lighting device in which the load is a high luminance discharge lamp, the load voltage fluctuates all the time depending on respective steps of starting and life of the discharge lamp, and it has been considered extremely difficult to keep such optimum conditions.
An object of the present invention is to provide a discharge lamp lighting device which can overcome the foregoing problem in the known arrangement, and which can constantly realize a highly efficient operation irrespective of conditions of the load, without causing the circuit efficiency to be varied due to any fluctuation in the load, when the device is applied to a discharge lamp lighting device.
Other objects and advantages of the present invention shall become clear as the description advances as detailed with reference to preferred embodiments shown in accompanying drawings.